The high output powers achievable with fiber lasers and amplifiers are largely due to the use of double-clad pumping and the ability of fibers to dissipate large amounts of heat as a result of their large surface-area-to-volume ratio. A conventional double-clad gain fiber includes a glass core surrounded by a glass inner cladding, which in turn is surrounded by a polymer outer cladding. For the active fibers used in high-power kW-class lasers and amplifiers, the core has a typical diameter of about 20 μm and is doped with an active ion, such as Yb3+. The glass forming the inner cladding has a slightly lower refractive index than the glass in the core so that the core acts as an optical waveguide for the light being amplified. For high-power applications, the diameter of the inner cladding may be several hundred microns.
The polymer outer cladding has a refractive index that is lower than the refractive index of the inner glass cladding. Typically the outer polymer cladding is several tens of microns thick and protects the fiber from being nicked or broken. The combination of the large diameter of the inner cladding and the difference between its refractive index and that of the polymer outer cladding turns the inner cladding into a “light pipe” capable of capturing and guiding a significant amount (e.g., kilowatts) of pump light from relatively low-intensity pump diodes.
As the pump light within the inner cladding traverses the core, it inverts the active ions in the core and produces optical gain in the fiber. The difference in the wavelength of the pump light and light being amplified results in quantum-defect heating. Quantum-defect heating generates heat at a rate equal to the absorbed pump power times the difference in the energy of a pump photon and a photon at the wavelength being amplified divided by the energy of a pump photon. Quantum-defect heating represents a lower bound to the rate of heat deposition, and in kW-class fiber lasers and amplifiers can approach 100 W m−1 of fiber.
With conventional double-clad fibers, the low-index polymer coating is the material most sensitive to a high thermal load. Since it serves the dual purpose of guiding the pump light and protecting the fiber, the materials and thicknesses that can be used are limited. Silicone has been used in the past. It can withstand relatively high temperatures, but suffers from degradation over time. Teflon (polytetrafluoroethylene) is difficult to apply. Although it has been used for passive fibers, the benefits for high-power active fibers have not been demonstrated. The most commonly used materials for the low-index outer cladding in active double-clad fibers are fluorinated acrylates. Fluorinated acrylates are easy to apply during the fiber drawing process and have excellent optical and mechanical properties. However, they have a low thermal conductivity (k≈0.24 W m−1 K−1) and degrade quickly at temperatures approaching 200° C. Long-term reliability for fibers with fluorinated acrylate claddings typically requires operation near or below 80° C.
The combination of the high heat load present in kW-class fiber lasers and amplifiers and the poor thermal properties of the polymer outer cladding makes heatsinking of the fiber critical. Conventionally, a high-power fiber is potted with an adhesive that provides the required heatsinking or covered with a thermal compound. As the power of fiber lasers and amplifiers continues to grow, however, the heatsinking provided by adhesives and thermal compounds cannot dissipate heat quickly enough to avoid damaging the optical fiber. Adhesives and thermal compounds also present creep, outgassing, and/or lifetime issues.
It has been suggested that the fibers be immersed in water (or other fluids) or embedded in bulk metal for heatsinking. Unfortunately, embedding a fiber in bulk metal is often not suitable for optical amplifiers used in coherent beam combining, which offers the potential to reach power levels and optical brightnesses that are otherwise unachievable with fiber lasers and amplifiers. For coherent optical beam combining, the phase noise on the output of the individual fibers should be kept low. Rigidly or semi-rigidly attaching the fiber to the heatsink (e.g., by embedding the fiber in the heatsink) provides a strong coupling of platform vibration and acoustics to the fibers, resulting in an unacceptable level of vibration-induced phase noise for coherent optical applications. (Even thermal compounds conventionally used for heatsinking are more rigid than is desired for such applications.) Immersing a fiber in fluid can introduce material compatibility issues, severely limit optical amplifier designs, and impose environmental constraints. Immersion or embedding also increases the size and weight of the system, making it less suitable for certain applications, including airborne applications.
For coherent optical applications, the best heatsinking achieved to date is obtained by creating a line contact (possibly two) between the fiber and the heatsink and designing the heatsink so that there is only a thin air layer between the fiber and the heatsink at other points on the fiber's surface. The thermal path through the air is a significant contribution to the thermal connectivity between the fiber and the heatsink. Typically, such a heatsink is implemented as a block of thermally conductive material (e.g., metal) with a U-shaped groove. The fiber is laid in the U-shaped groove and can slide back and forth within the U-shaped groove.
At sea level, when the optical fiber is placed in a reasonably tight U-shaped groove in a metal heatsink, the heat load in the fiber can be up to about 40 W per meter without the fiber overheating, corresponding to a typical output power of about 1.5 kW. At high altitudes, where there is less air, or for space-based application, the output power of the fibers may be much more limited. But for many of these applications, it is desirable to operate the individual fibers at output powers well in excess of 1.5 kW. Unfortunately, conventional polymer-clad fibers cannot operate at these optical power levels because there is not a suitable way to dissipate heat from the polymer claddings.